Tentative implementation of string slices (hidden under the flag --string-slices).

src/x64/lithium-x64.h

Index: src/x64/lithium-x64.h
diff --git a/src/x64/lithium-x64.h b/src/x64/lithium-x64.h
index 12bdfb28aed4bb1966270cf28ef05c5433e91bf4..4a3ab39515ae65eeb04c75a9fabc1109667b6ad4 100644
--- a/src/x64/lithium-x64.h
+++ b/src/x64/lithium-x64.h
@@ -1683,11 +1683,12 @@ class LStringAdd: public LTemplateInstruction<1, 2, 0> {
 };
 
 
-class LStringCharCodeAt: public LTemplateInstruction<1, 2, 0> {
+class LStringCharCodeAt: public LTemplateInstruction<1, 2, 1> {
  public:
-  LStringCharCodeAt(LOperand* string, LOperand* index) {
+  LStringCharCodeAt(LOperand* string, LOperand* index, LOperand* temp) {
     inputs_[0] = string;
     inputs_[1] = index;
+    temps_[0] = temp;
   }
 
   DECLARE_CONCRETE_INSTRUCTION(StringCharCodeAt, "string-char-code-at")